I. Field of the Invention
The present invention relates to digital wireless communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for constructing a temporary list of neighboring base stations in a wireless communication device.
II. Description of the Related Art
In the field of wireless communications, several technology-based standards exist for controlling communications between a mobile station, such as a cellular telephone, Personal Communication System (PCS) handset, or other remote subscriber communication device, and a wireless base station. These include both digital-based and analog-based standards. For example, among the digital-based cellular standards are the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) Interim Standard IS-95 series including IS-95A and IS-95B, entitled "Mobile Station--Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System." Similarly, among the digital-based PCS standards are the American National Standards Institute (ANSI) J-STD-008 series, entitled "Personal Station--Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems." Other non-CDMA based digital standards include the time-division multiple access (TDMA) based Global System for Mobile Communications (GSM), and the U.S. TDMA standard TIA/EIA IS-54 series.
The spread spectrum modulation technique of CDMA has significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, issued Mar. 31, 1992, entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM", and U.S. Pat. No. 5,109,390, issued Apr. 28, 1992, entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", both assigned to the assignee of the present invention and incorporated by reference herein.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", Ser. No. 07/433,031 now U.S. Pat. No. 5,056,109, filed Nov. 7, 1989, also assigned to the assignee of the present invention. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
The aforementioned patents all describe the use of a pilot signal used for acquisition in a CDMA wireless communication system. At various times when a wireless communication device such as a cellular or PCS telephone is energized, it undertakes an acquisition procedure which includes, among other things, searching for and acquiring the pilot channel signal from a base station in the wireless communication system. For example, demodulation and acquisition of a pilot channel in a CDMA system is described in more detail in copending U.S. patent application Ser. No. 08/509,721 now U.S. Pat. No. 5,805,648, entitled "METHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM," assigned to the assignee of the present invention and incorporated herein by reference. When more than one pilot channel can be acquired by the wireless communication device, it selects the pilot channel with the strongest signal. Upon acquisition of the pilot channel, the wireless communication device is rendered capable of acquiring additional channels from the base station that are required for communication. The structure and function of these other channels is described in more detail in the above referenced U.S. Pat. No. 5,103,459 and will not be discussed in detail herein.
Once the wireless communication device has acquired the pilot channel, it may also start demodulating the sync channel. The sync channel carries cell site identification, the cell site pilot PN carrier offset, the paging channel data rate, and system time. With this information, the wireless communication device is capable of demodulating the paging channel. The paging channel is used for transmission of control information and pages from a base station to the wireless communication device. One of the control messages transmitted by the base station to the wireless communication device is the Neighbor List Message, which contains configuration information to speed handoff to surrounding base stations.
An example of one such neighbor list message is the "Extended Neighbor List Message" of J-STD-008. When the base station sends an Extended Neighbor List Message to the mobile station, it uses the format of Table I.
TABLE I ______________________________________ Field Length (bits) ______________________________________ MSG.sub.-- TYPE (`00001110`) 8 PILOT.sub.-- PN 9 CONFIG.sub.-- MSG.sub.-- SEQ 6 PILOT.sub.-- INC 4 Zero or more occurrences of the following record: NGHBR.sub.-- CONFIG 3 NGHBR.sub.-- PN 9 SEARCH.sub.-- PRIORITY 2 FREQ.sub.-- INCL 1 NGHBR.sub.-- BAND 0 or 5 NGHBR.sub.-- FREQ 0 or 11 RESERVED 0-7 (as needed) ______________________________________
The above table is taken from Section 3.7.2.3.2.14 of J-STD-008, and indicates the various fields transmitted in an exemplary Extended Neighbor List Message. Of particular concern to the present invention is the NGHBR.sub.-- CONFIG field. The base station includes one occurrence of the NGHBR.sub.-- CONFIG record for each neighboring base station. In J-STD-008, the value of the NGHBR.sub.-- CONFIG field is set according to the following table:
TABLE II ______________________________________ Value (binary) Neighbor Configuration ______________________________________ 000 The neighbor base station has the same number of frequencies having paging channels as the current base station and has a CDMA frequency assignment corresponding to this CDMA frequency assignment with the same number of paging channels. 001 The neighbor base station has the same number of frequencies having paging channels as the current base station and has a CDMA frequency assignment corresponding to this CDMA frequency assignment with a different number of paging channels. 010 The neighbor base station may have a different number of frequencies having paging channels as the current base station. 011 The neighbor base station configuration is unknown. ______________________________________
In simple terms, the value "000" indicates that the neighboring base station has the same configuration as the current base station. The value "001" indicates that the neighboring base station has the same configuration as the current base station except that it has a different number of paging channels. The value "010" indicates that the neighboring base station has a different number of CDMA frequency assignments and may have a different number of paging channels. The value "011" indicates that the neighbor has an unknown configuration.
Thus, according to J-STD-008 and IS-95, the mobile station is given an indication of whether the neighboring base station is on the same frequency assignment and has the same number or a different number of paging channels. This gives the mobile station enough information to begin the process of demodulating the neighboring base station's paging channel immediately upon acquiring its pilot channel, rather than having to wait for further information. For example, the mobile station may keep a table of all the neighbors that were passed to it in the neighbor list message or extended neighbor list message, plus all of the neighbors that it detected during its own independent searching. Such a table might include entries similar to Table III below. It should be noted that Table III is merely exemplary, and other neighbor lists may contain more or different fields representing other parameters of the neighboring base stations. The first column, "Reference", is provided merely for convenience in referring to the rows and may not be present in the wireless communication device.
TABLE III ______________________________________ PN Offset Reference (chips) Frequency NGHBR.sub.-- CONFIG ______________________________________ A 12 f(1) 000 B 24 f(1) 001 C 48 f(1) 000 D 12 f(2) 010 E 24 f(2) 011 ______________________________________
This table is accessed by the wireless communication device when it needs to perform an idle handoff as a result of the neighboring base station's (the "target" base station's) pilot signal strength exceeding the current base station's pilot signal strength by a predetermined amount. An "idle handoff" is a handoff that occurs when the wireless communication device is in "idle" mode, monitoring its assigned paging channel, and not involved in an active call. Handoffs are discussed in more detail in the above-referenced U.S. Pat. No. 5,101,501.
The information retrieved from the neighbor list table helps speed up the idle handoff process. However, both IS-95 section 6.6.2.1.4 and J-STD-008 section 2.6.2.1.4 require that if the neighboring base station is not listed in the Neighbor List Message (of IS-95) or Extended Neighbor List Message (of J-STD-008) of the current base station, the wireless communication device shall perform the idle handoff operation using the same procedure as for a pilot in the list with the NGHBR.sub.-- CONFIG field set to "011". In other words, when handing off to a target base station which was not reported to the wireless communication device by the current base station, the wireless communication device must treat the target base station as being of "unknown" configuration.
If the configuration of the target base station is unknown, the wireless communication device must restart an extensive re-acquisition process, disregarding most or all of its prior knowledge of the frequency assignment, pilot channel, and paging channels of the target base station. Such an extensive re-acquisition will be referred to herein as a "full re-acquisition." Since the time for a full re-acquisition may be on the order of several seconds, this can negatively affect the performance of the wireless communication device if the wireless communication device is busy re-acquiring the target base station rather than monitoring its paging channel for incoming control messages or pages. Under dynamic conditions where there are many base stations in a small geographical area, or where the wireless communication device is traveling quickly between coverage areas of neighboring base stations, the wireless communication device must perform many idle handoffs quickly in order to maintain adequate monitoring of its assigned paging channel and avoid missing incoming pages.
For example, consider the situation where the wireless communication device has acquired a first system, and has received an Extended Neighbor List Message for that system. In response to the Extended Neighbor List Message received, the wireless communication device may build a neighbor list similar to that of TABLE III, containing system parameter information for each neighboring base station (referenced, for convenience, as base stations A-E) listed in-the Extended Neighbor List Message. Suppose the wireless communication device then detects the pilot signal of one of the base stations in the neighbor list, for example, base station B. When the received pilot signal strength of base station B becomes sufficiently stronger than that of the current base station, the wireless communication device will execute an idle handoff to base station B. Then, if the wireless communication device suddenly detects a sufficiently strong pilot signal of base station C, it will immediately execute a subsequent idle handoff to base station C. In a rapidly changing environment, this handoff may occur before the wireless communication device receives an Extended Neighbor List Message from base station B.
If no Extended Neighbor List Message has been received from base station B, then the wireless communication device would have to perform a full re-acquisition of base station C based on the rule as stated above that hen handing off to a target base station which was not reported to the wireless communication device by the current base station, the wireless communication device must treat the target base station as being of "unknown" configuration. In this example, the current base station, B, was unable to report the presence of base station C to the wireless communication device because of the short time that the wireless communication device was monitoring base station B's pilot channel. As such, the wireless communication device was required to treat base station C as being of "unknown" configuration, and perform a full re-acquisition of base station C. This may often be the case in a dynamically changing environment where back-to-back idle handoffs occur quickly.
Clearly, it would be advantageous for the wireless communication device to avoid having to perform a lengthy re-acquisition, and instead spend its time monitoring its assigned paging channel for incoming pages.
If the wireless communication device is in an environment where many back-to-back idle handoffs are occurring quickly, it may miss incoming pages due to the large percentage of time it spends performing full re-acquisition of the target base station rather than monitoring the paging channel.